Reading text files is easy using the Get-Content cmdlet:
$text = Get-Content $env:windirwindowsupdate.log
However, Get-Content reads the file line by line so you will most likely get back an array. This can cause problems when you really need the entire file content as text.
$text.GetType().FullName
If you need the complete text in one piece, try combining Get-Content with Out-String, which converts the array into one single string:
$text = Get-Content $env:windirwindowsupdate.log -Delimiter 'doesnotexist'
$text.GetType()
$text.length